Preheat air fryer to 320°F (160°C).
Grease and line a 6-inch round cake pan with parchment paper.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate bowl, whisk the milk, melted butter, egg, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, whisking until the batter is smooth.
Pour the batter into the prepared cake pan.
Place the pan into the air fryer basket and cook at 320°F for 25 minutes or until a toothpick inserted in the center comes out clean.
Carefully remove the pan and allow the cake to cool for 10 minutes before removing from the pan.